A critical part of any advanced semiconductor integrated circuit involves the one or more metallization levels used to contact and interconnect the active semiconductor areas, themselves usually residing in a fairly well defined crystalline silicon substrate. Although it is possible to interconnect a few transistors or other semiconductor devices, such as memory capacitors, within the semiconductor level, the increasingly complex topology of multiply connected devices soon necessitates another level of interconnect. Typically, an active silicon layer with transistors and capacitors formed therein is overlaid with a dielectric layer, for example, silicon dioxide. Contact holes are etched through the dielectric layer to particular contacting areas of the silicon devices. A metal is filled into the contact holes and is also deposited on top of the dielectric layer to form horizontal interconnects between the silicon contacts and other electrical points. Such a process is referred to as metallization.
A single level of metallization may suffice for simple integrated circuits of small capacity. However, dense memory chips and especially complex logic devices require additional levels of metallization since a single level does not provide the required level of interconnection between active areas. Additional metallization levels are achieved by depositing over the previous metallized horizontal interconnects another level of dielectric and repeating the process of etching holes, now called vias, through the dielectric, filling the vias and overlaying the added dielectric layer with a metal, and defining the metal above the added dielectric as an additional wiring layer. Very advanced logic device, for example, fifth-generation microprocessors, have five or more levels of metallization.
Conventionally, the metallized layers have been composed of aluminum and aluminum-based alloys additionally comprising at most a few percent of alloying elements such as copper and silicon. The metallization deposition has typically been accomplished by physical vapor deposition (PVD), also known as sputtering. A conventional PVD reactor 10 is illustrated schematically in cross section in FIG. 1, and the illustration is based upon the Endura PVD Reactor available from Applied Materials, Inc. of Santa Clara, Calif. The reactor 10 includes a vacuum chamber 12 sealed to a PVD target 14 composed of the material to be sputter deposited on a wafer 16 held on a heater pedestal 18. A shield 20 held within the chamber protects the chamber wall 12 from the sputtered material and provides the anode grounding plane. A selectable DC power supply 22 biases the target negatively to about −600 VDC with respect to the shield 20. Conventionally, the pedestal 18 and hence the wafer 16 is left electrically floating.
A gas source 24 of sputtering working gas, typically chemically inactive argon, supplies the working gas to the chamber through a mass flow controller 26. A vacuum system 28 maintains the chamber at a low pressure. Although the base pressure can be held to about 10−7 Torr or even lower, the pressure of the working gas is kept between about 1 and 1000 mTorr. A computer-based controller 30 controls the reactor including the DC power supply 22 and the mass flow controller 26.
When the argon is admitted into the chamber, the DC voltage between the target 14 and the shield 20 ignites the argon into a plasma, and the positively charged argon ions are attracted to the negatively charged target 14. The ions strike the target 14 at a substantial energy and cause target atoms or atomic clusters to be sputtered from the target 14. Some of the target particles strike the wafer 16 and are thereby deposited on it, thereby forming a film of the target material.
To provide efficient sputtering, a magnetron 32 is positioned in back of the target 14. It has opposed magnets 34, 36 creating a magnetic field within the chamber in the neighborhood of the magnets 34, 36. The magnetic field traps electrons, and for charge neutrality, the ion density also increases to form a high-density plasma region 38 within the chamber adjacent to the magnetron 32.
With the continuing miniaturization of integrated circuits, the demands upon the metallization have increased. Many now believe that aluminum metallization should be replaced by copper metallization. Murarka et al. provide a comprehensive review article on copper metallization in “Copper metallization for ULSI and beyond,” Critical Reviews in Solid State and Materials Science, vol. 10, no. 2, 1995, pp. 87–124. Copper offers a number of advantages. Its bulk resistivity is less than that of aluminum, 1.67 μΩ-cm vs. 2.7 μΩ-cm for pure material, and any reduction in resistivity offers significant advantages as the widths and thicknesses of the metallization interconnects continue to decrease. Furthermore, a continuing problem with aluminum metallization is the tendency of aluminum atoms in an aluminum interconnect carrying a high current density to migrate along the interconnect, especially away from hot spots, in a process called electromigration. Any excessive amount of such migration will break an aluminum interconnect and destroy the integrated circuit. Copper-based alloys exhibit significantly reduced levels of electromigration.
Copper metallization is an unproven technology and is acknowledged to offer difficulties not experienced with the conventional aluminum metallization. However, it may afford ways to circumvent problems inherent in aluminum metallization.
One problem inherent in conventional sputtering is that it is performed in a fairly high pressure of the inert working gas, such as argon. However, the argon environment presents two problems. First, it is inevitable that some argon ions are deposited on the substrate and incorporated into the sputter deposited aluminum. Although the effect of these usually inactive argon ions is not precisely known, it is probably not positive.
The second problem relates to the directionality of the sputtered particles. Sputtering to fill holes relies at least in part on the sputtered particles being ballistically transported from the target to the wafer, that is, without scattering from the initial course. The ballistic trajectories allow the sputtered particles to arrive at the wafer nearly perpendicularly to the wafer's surface and thus to deeply penetrate into any aperture. However, the typical sputtering is performed in an argon ambient of from 1 to 100 mTorr. Such a high pressure means that there is a significant probability that the aluminum sputter particles will collide with the argon atoms and thus be deflected from their ballistic paths. Accordingly, low-pressure sputtering is believed to provide better hole filling for deep vias. However, low pressure is generally equated with low deposition rates so that reducing the pressure is not a favored method for better directionality. Furthermore, a minimum pressure approaching 1 mTorr is required to support a plasma in the usual configuration of FIG. 1.
High-density plasma (HDP) sputter reactors are being actively developed and are approaching commercialization. One of the advantages of HDP sputtering is that a sizable fraction of the sputtered particles are ionized during their travel toward the substrate. Then, the pedestal supporting the wafer can be selectively biased by an RF source to create a DC self-bias with respect to the positively charged plasma. As a result, the wafer can be biased negatively with respect to the plasma (−20V being a typical value), and the positively charged sputtered ions are accelerated from the generally neutral plasma toward the substrate. The added velocity provides a highly directional flux normal to the plane of the substrate, thus reaching deeply into holes of high aspect ratios. Nulman in European Patent Publication 703,598-A1 discloses inserting a negatively biased grid between the substrate and the HDP source using argon working gas.
There has been much recent interest in the PVD deposition of copper films using sustained self-sputtering (SSS), for example, as disclosed by Posadowski et al. in “Sustained self-sputtering using a direct current magnetron source,” Journal of Vacuum Science and Technology, A, vol. 11, no. 6, 1993, pp. 2980–2984. No working gas is used in sustained self-sputtering, at least after the plasma has been ignited. Instead, a sufficient number of the atoms sputtered from the target are ionized and are then attracted back to the target at sufficiently high energy to serve as the sputtering ions in place of the more typical argon ions.
The condition for achieving self-sustained sputtering, which is observed only with some target materials under special conditions, may be expressed asα·β·Sm=1,  (1)where α is the ionization fraction of the atoms sputtered from the target, β is the ratio of sputtered atoms that return to the target, and Sm is the self-sputtering yield, that is, the number of copper atoms in the case of a copper target that are sputtered from the target by one returning copper atom. The ionization fraction and the return ratio depend upon the material and the current density, but they are necessarily less than unity. Generally the product αβ increases at high current density. Hence, a large value of the self-sputtering yield is crucial for sustained self-sputtering, and a high current density is also important. The conventional metallization material Al and other metals used with Al hole filling, viz., Ti, Mo, W, and Ta, have sub-unity self-sputtering yields, thus precluding their use in sustained self-sputtering. However, Cu has an acceptable value of self-sputtering, as do Pd, Pt, Ag, and Au.
Asamaki et al. have reported the SSS deposition of copper in “Copper self-sputtering by planar magnetron,” Japanese Journal of Applied Physics, vol. 33, pt. 1, no. SA, 1994, pp. 2500–2503 and in “Filling of sub-μm through holes by self-sputter deposition,” Japanese Journal of Applied Physics, vol. 33, pt. 1, no. 8, 1994, pp. 4566–4569. They reported in the last reference very good bottom coverage in 0.4 μm holes having aspect ratios of about 3.
However, the known SSS work has been of an experimental nature and several difficult problems need to be addressed before sustained self-sputtering can be commercialized for the mass integrated circuit market.
Most new semiconductor fabrication equipment is being developed for wafer sizes of 200 mm, and, in view the burgeoning developments for fabricating 300 mm wafers, new technology such as copper SSS needs to be shown to be usable at 200 mm and believed to be scalable to 300 mm. Even for 200 mm wafers, the typical target diameter for commercial PVD reactors is about 325 mm.
Achieving uniform PVD deposition over 200 mm at a reasonable deposition rate even for conventional PVD of aluminum has been challenging. Parker in U.S. Pat. No. 5,242,566 and Tepman in U.S. Pat. No. 5,320,728 disclose magnetrons having a generally linear array of magnets arranged along the outline of a kidney shape and the array being rotated at the back of the target about a point either within the kidney shape or a point having both halves of a diameter passing through the kidney shape. This magnet array has a size of about the size of the wafer for a 325 mm target over a 200 mm wafer. The large size of the magnet array is consistent with the trend to larger magnets. However, we have tested the conventional Tepman design for sustained self-sputtering of copper but could not achieve sustained self-sputtering.
A conventional Tepman magnetron produces a magnetic field within the chamber of about 200 gauss. This magnetic intensity is somewhat low for sustained self-sputtering which requires a high plasma density, which depends at least in part on a high magnetic intensity. However, the extended arrangement of Tepman presents some more fundamental problems.
First, the electrons in a high-density (HDP) plasma tend to quickly diffuse to the sides. In the kidney arrangement of Tepman, as with other linear magnetic arrays, the linearly concentrated magnetic field distribution is surrounded on two sides by low-field regions. That is, the HDP electrons tend to diffuse away from the HDP region and be lost, thereby reducing the plasma density from the high levels required for SSS.
Secondly, the region of relatively high magnetic field in the Tepman arrangement extends over a relatively large fraction of the target. Although this large coverage may promote uniformity, it means that for a given amount of electrical power applied to the target the large area reduces the power density into the high-density plasma. For large commercial PVD reactors, the amount of target power required with the Tepman magnetron to sustain self-sputtering becomes excessive.
Reports of sustained self-sputtering of copper seem to indicate that a current density of about 200 mA/cm2 is required. For 50 mm targets, the DC power applied to the target for successful SSS with a stationary magnet in fairly conventional PVD reactors has been 6 kW; for 100 mm targets, 16 kW; for 200 mm targets, 20 kW. The scaling trend indicates that for 200 mm and 300 mm wafers, which require significantly larger targets for uniformity, the DC bias powers will be 35 kW to 50 kW, assuming a conventionally sized magnetron of the prior art. These power levels are considered to be impractical in commercial equipment. A 12 kW power supply is considered to be economically advantageous, and a 20 kW one to be marginally acceptable.
For these reasons, it is desirable to achieve sustained self-sputtering at a reduced power level even for large targets. It is further desirable to provide additional directional control of sputtered ions.